1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) apparatus having a SAW device that includes a piezoelectric substrate having pyroelectric properties and which is housed in a package. More particularly, the present invention relates to an edge-reflection-type SAW apparatus having a mechanism for preventing deterioration in performance or electrode damage due to a pyroelectric effect.
2. Description of the Related Art
When a surface acoustic wave (SAW) apparatus including a piezoelectric substrate having pyroelectric properties experiences a rapid change in temperature, a charge caused by the pyroelectric effect is generated on the piezoelectric substrate. This might result in discharging between a plurality of electrodes disposed on the piezoelectric substrate, causing damage to the electrodes. If the piezoelectric substrate is made of a piezo-ceramic, the piezoelectricity of the piezoelectric substrate is significantly reduced.
In order to solve the foregoing problems caused by the pyroelectric effect, a variety of proposals have been made. A SAW apparatus 101 shown in FIG. 4 is disclosed in Japanese Utility Model Publication No. 2-15388. In this apparatus 101, an interdigital transducer (IDT) 103, and reflectors 104 and 105 are disposed on the top of a piezoelectric substrate 102. A short-circuited electrode 106 having a rectangular band shape is disposed on the top of piezoelectric substrate 102 along its outer periphery. The technique described in this publication prevents dust that is scattered during manufacturing from adhering to electrodes such as the IDT 103 because of static electricity or pyroelectric charge generated during the heating step when a SAW device 101 is bonded to a package.
A SAW device 111 shown in FIG. 5 is disclosed in Japanese Unexamined Patent Application Publication No. 56-16312. A piezoelectric substrate 112 is polarized in the direction indicated by an arrow P. IDTs 113 and 114 are disposed on the top of the piezoelectric substrate 112. In order to prevent deterioration in piezoelectricity due to the pyroelectric effect in the case where the piezoelectric substrate 112 is made of a piezo-ceramic, conductive members 115 and 116 are disposed on ends 112a and 112b of the piezoelectric substrate 112, respectively, so as to extend perpendicular to the polarization direction. The conductive members 115 and 116 are electrically connected to a lead 117. Japanese Unexamined Patent Application Publication No. 56-37723 also discloses a SAW device having the same construction as in Japanese Unexamined Patent Application Publication No. 56-16312.
However, there are problems associated with the above-described techniques of the related art.
The SAW apparatus disclosed in Japanese Utility Model Publication No. 2-15388 requires the band-shaped short-circuited electrode 106 disposed around the periphery of the piezoelectric substrate 112. Therefore, the SAW apparatus of this type cannot be implemented as, for example, an edge-reflection-type SAW apparatus 121 shown in FIG. 6 which uses a Shear Horizontal (xe2x80x9cSHxe2x80x9d) surface acoustic wave. The edge-reflection-type SAW apparatus 121 requires electrode fingers along facing ends 122a and 122b of a piezoelectric substrate 122 or in the vicinity of the ends 122a and 122b. Therefore, the short-circuited electrode described in Japanese Utility Model Publication No. 2-15388 cannot be located along the outer periphery of the piezoelectric substrate 122.
The SAW apparatus disclosed in Japanese Unexamined Patent Application Publication No. 56-16312 or No. 56-37723 requires the steps of, first, forming an electrode on the top of a mother piezoelectric substrate, cutting the piezoelectric substrate 112 therefrom by dicing, and then forming the conductive members 115 and 116 on the edges 112a and 122b of the piezoelectric substrate 112. Therefore, an additional manufacturing step is necessary, thereby increasing the cost, time and difficulty of manufacturing.
In order to overcome the above-described problems with the related art, preferred embodiments of the present invention provide an edge-reflection-type SAW apparatus that eliminates the adverse effects of a pyroelectric charge due to a rapid change in temperature without requiring any additional manufacturing step.
According to a preferred embodiment of the present invention, a SAW apparatus includes a piezoelectric substrate having pyroelectric properties and having first and second main surfaces opposing each other, first and second sides opposing each other, and first and second ends opposing each other, at least one IDT disposed on the first main surface of the piezoelectric substrate for propagating surface acoustic waves in the direction that is substantially perpendicular to the first and second sides, the surface acoustic waves being reflected by the first and second sides, a SAW device disposed on the first main surface of the piezoelectric substrate, the SAW device including a first pyroelectric charge cancellation electrode in the vicinity of an edge defined by the first end and the first main surface, and a second pyroelectric charge cancellation electrode in the vicinity of an edge defined by the second end and the first main surface, and a package which houses the SAW device and which includes a plurality of electrodes which are electrically connected to the SAW device. The first and second pyroelectric charge cancellation electrodes are electrically connected via the electrodes on the package rather than directly on the piezoelectric substrate.
The first and second pyroelectric charge cancellation electrodes are not located on the first and second sides of the piezoelectric substrate, respectively, but in the vicinity of the edge defined by each of the first and second ends and the first main surface. The first and second pyroelectric charge cancellation electrodes are located in a region other than in or around the SAW propagation pass, and therefore, the performance of the SAW apparatus is not influenced by the pyroelectric charge cancellation electrodes. Since the first and second pyroelectric charge cancellation electrodes are electrically connected via the electrodes on the package, the pyroelectric charges having opposite polarities which are generated at both ends in the polarization direction are cancelled. Therefore, if temperature rapidly changes, pyroelectric charges do not damage the electrodes or deteriorate the performance of the SAW device on the piezoelectric substrate.
According to another preferred embodiment of the present invention, the SAW device is disposed on the piezoelectric substrate, and has a plurality of electrode patterns connected to the at least one IDT, the plurality of electrode patterns are electrically connected to the plurality of electrodes on the package, and the first and second pyroelectric charge cancellation electrodes are electrically connected to different electrodes of the electrodes on the package via different electrode patterns of the electrode patterns on the piezoelectric substrate, the different electrodes on the package being electrically connected within the package. The first and second pyroelectric charge cancellation electrodes are electrically connected to different electrodes on the package via the electrode patterns which are used to electrically connect the IDTs to the electrodes on the package. Therefore, there is no need for any additional electrically connecting member, such as a bonding wire, for electrically connecting the first and second pyroelectric charge cancellation electrodes to the electrodes on the package.
The first and second pyroelectric charge cancellation electrodes may be arranged along the edge defined by each of the first and second ends and the first main surface. Alternatively, the first and second pyroelectric charge cancellation electrodes may be separated from the edges.
In another preferred embodiment of the present invention, the SAW device may be electrically connected to the plurality of electrodes on the package by a plurality of bonding wires. The first and second pyroelectric charge cancellation electrodes may be bonded to different electrodes on the package by the bonding wires. Otherwise, if the first and second pyroelectric charge cancellation electrodes are electrically connected to the electrode patterns connected to the IDTs, the electrode patterns may be electrically connected to the electrodes on the package by the bonding wires.
In another preferred embodiment of the present invention, the piezoelectric substrate is made of a piezo-ceramic. A piezo-ceramic significantly generates the pyroelectric effect due to a change in temperature. The SAW apparatus constructed according to various preferred embodiments of the present invention effectively prevents the adverse effects of the pyroelectric effect which is susceptible to the piezo-ceramic.
In another preferred embodiment of the present invention, a communication apparatus includes the SAW apparatus in accordance with other preferred embodiments of the present invention, with the SAW apparatus defining a bandpass filter of the communication apparatus. As a result, the communication apparatus eliminates the adverse effects of the pyroelectric effect which is generated on the piezoelectric substrate due to a rapid change in temperature. Therefore, the risk of electrode damage or deterioration in performance of the bandpass filter is minimized. The communication apparatus is therefore highly reliable and provides stable performance.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments thereof with reference to the attached drawings.